NATO is looking to counter Russia's military build-up in Crimea, which Moscow annexed from Ukraine in 2014, and in the Black Sea, which is strategically important for both East and West given its energy reserves and closeness to the Middle East.

"There are some very valuable discussions under way among the allies who live on the Black Sea ... of more closely integrating their naval forces and operations," NATO's Deputy Secretary General Alexander Vershbow said in a visit to Sofia, mentioning the three NATO allies by name.

NATO already patrols the Black Sea, but Vershbow said that by the time Western leaders meet for a NATO summit in July, allies could have "an enhanced presence" in the area as part of plans to move troops on rotation into the Baltics and Poland.

"We need to consider a more persistent NATO military presence in the region, with a particular focus on our maritime capabilities," he said.

Worried since the seizure of Crimea and pro-Russian rebel operations in eastern Ukraine that Moscow could exert pressure on Poland or destabilize the Baltic states - perhaps by fomenting unrest in their Russian minority populations - the West wants to bolster defenses on its eastern flank without provoking the Kremlin by stationing large forces permanently.